Freud's essay "Medusa's Head" (1922), printed here, is included in a group of essays entitled Sexuality and the Psychology of Love.  In this essay, he lays the framework his significant contribution to a body of criticism surrounding the Medusa Myth. comic version of medusa

Medusa's Head

We have not often attempted to interpret individual mythological themes, but an interpretation suggests itself easily in the case of the horrifying decapitated head of Medusa.
    To decapitate = to castrate.  The terror of Medusa is thus a terror of castration that is linked to the sight of something.  Numerous analyses have made us familiar with the occasion for this: it occurs when a boy, who has hitherto been unwilling to believe the threat of castration, catches sight of the female genitals, probably those of an adult, surrounded by hair, and essentially those of his mother.
    The hair upon Medusa's head is frequently represented in works of art in the  form of snakes, and these once again are derived form the castration complex.  It is a remarkable fact that, however frightening they may be in themselves, they nevertheless serve actually as a mitigation of the horror, for they replace the penis, the absence of which is the cause of the horror.  This is a confirmation of the technical rule according to which a multiplication of penis symbols signifies castration.
    This sight of Medusa's head makes the spectator stiff with terror, turns him to stone.  Observe that we have here once again the same origin from the castration complex and the same transformation of affect!  For becoming stiff means an erection. Thus in the original situation it offers consolation to the spectator: he is still in possession of a penis, and the stiffening reassures him of the fact.
    This symbol of horror is worn upon her dress by the virgin goddess Athena.  And rightly so, for thus she becomes a woman who is unapproachable and repels all sexual desire-- since she displays the terrifying genitals of the Mother.  Since the Greeks were in the main strongly homosexual, it was inevitable that we should find among them a representation of woman as a being who frightens and repels because she is castrated.
    If Medusa's head takes the place of a representation of the female genitals, or rather if it isolates their horrifying effects from the pleasure-giving ones, it may be recalled that displaying the genitals is familiar in other connections as an apotropaic act.  What arouses  horror in oneself will produce that same effect upon the enemy against whom one is seeking to defend oneself.  We read in Rablais of how the Devil took to flight when the woman showed her vulva.
    The erect male organ also has an apotropaic effect, but thanks to another mechanism.  To display the penis (or any of its surrogates) is to say: "I am not afraid of you.  I defy you.  I have a penis."  Here, then, is another way of intimidating the Evil Spirit.
    In order seriously to substantiate this interpretation it would be necessary to investigate the origin of this isolated symbol of horror in Greek mythology as well as parallels to it in other mythologies.

Freud, S. (1963) Sexuality and the Psychology of Love. NY: Collier.  (pp.212-213).
["Das Medusenhaupt."  First published posthumously. Int. Z. Psychoanal. Imago, 25 (1940), 105; reprinted Ges. W., 17,47.  The manuscript is dated May 14, 1922, and appears to be a sketch for a more extensive work.  Translation , reprinted from Int. J. Psychoanal.,22 (1941), 69; by James Strachey.] complete bibliography